The present invention relates to a rotor machine, and particularly to a helical screw rotor machine which comprises a housing in which at least one rotor provided with trunnions is enclosed in a working space that includes an inlet port and an outlet port, wherein the working space is delimited by a low pressure end-section, a high pressure end-section and a barrel section extending between the end-sections, wherein the trunnions extend into bearings disposed in the end-sections, and wherein at least one of the trunnions extends through an associated end-section and presents an axially projected thrust surface in a delimited chamber which contains means for creating a force that acts axially on the thrust surface.
When such helical screw rotor machines are designed to function as compressors, the working medium is compressed to a higher pressure level, whereas when such machines are designed to expand the working medium, the working medium is expanded from an elevated pressure level. For the sake of simplicity, solely the former case will be dealt with, i.e. the case when the machine functions as a compressor, although the following discussion also applies to the same degree with respect the case when the machine functions as an expander.
In a helical screw rotor compressor, the working medium is compressed in V-shaped working chambers. During a filling phase, each working chamber is in communication with an inlet port disposed at the low pressure end. When communication with the inlet port has been broken, the volume of the working chamber decreases as a result of said chamber being moved in a direction towards the high pressure end by rotation of the rotors, thereby compressing the working medium enclosed in the working chamber. When the working chamber is moved axially towards the high pressure end to an extent such as to begin to communicate with the outlet port, an emptying phase commences, during which continued reduction in the volume of the working chamber forces the working medium out through the outlet port at an elevated pressure level. Thus, the rotors are exposed to a higher pressure at their high pressure end than at their low pressure end, meaning that each rotor is subjected to thrust in a direction towards the low pressure end. These thrust forces are taken up by thrust bearings mounted in one or both end sections.
Some working medium will also leak out from the high pressure end around the trunnions and enter the bearing chamber in the high pressure end-section. In order to avoid the build-up of high pressure, on a level with the outlet pressure, in the bearing chamber, said chamber is normally provided with a relief channel that leads the working medium back to a closed working chamber in which the pressure at one level is slightly higher than the inlet pressure. This channel is also intended to allow oil to circulate through the rotor bearings. As a result, the pressure in the bearing chamber will be on the level of the pressure in said closed working chamber. This pressure exerts a force on the end surfaces of the rotor trunnions, which is also directed towards the low pressure end of the compressor.
The axial forces acting on the rotors as a result of the pressure difference between the low pressure end and the high pressure end vary in magnitude during the compression stage, and said forces are distributed differently on the two rotors as a result of the mutual contact of the rotors between the flank surfaces of the lobes and the grooves. This distribution of the axially acting forces also varies during the compression stage. The force acting axially on each rotor will therefore be pulsating. When the compressor works under full load, the axially acting forces caused by the working medium are sufficiently large for the resultant force on each rotor to always remain directed towards the low pressure end, even if the magnitude of the force varies.
A compressor of this kind is conventionally relieved of load by throttling the inlet pressure significantly, down to about 0.1 bar, and, at the same time, lowering the pressure on the outlet side to about half the outlet pressure at full load.
When the compressor is driven free from load, the axial forces acting on the rotors in a direction towards the low pressure end, as described above, will be smaller, partly because the pressure difference between the outlet pressure and the inlet pressure is smaller and partly because the pressure in the bearing chamber of the high pressure end-section is lower. In this regard, there is a risk that these axial forces will not be large enough to ensure that the resultant force on each rotor will constantly be directed towards the low pressure end because of the above described force pulsations. The resultant axial force on a rotor can therefore change sign instantaneously, and act in a direction towards the high pressure end. This will result in vibration of one or both rotors in the axial direction. Rattling then occurs as the flanks of the rotors hit each other. These impacts damage the rotors and reduce the length of life of the bearing.
The rattling problem can be overcome by applying an axial force on one or both rotors in a direction towards the low pressure end of the compressor, while the problem caused by the high load on the thrust bearing of a rotor when the rotor is influenced axially from the high pressure side can be overcome by applying a force axially on one or both rotors in a direction towards the high pressure side of the machine.
An object of the present invention is to relieve the thrust bearings of helical rotor machines of the large axial forces in a simple and reliable fashion, or to counteract rattling with partial loads by applying to the rotors an axially directed force that acts in the opposite or same direction as the gas pressure acting through compression, respectively.
This has been achieved in accordance with the invention with helical screw rotor machines by placing around the one trunnion which has the axial thrust surface, with a close fit, a casing which has a generally circular, cylindrical outer surface and which is freely disposed in the chamber and has an outer end which is closed by a bottom wall that has a hole in its center, wherein the casing is rotatably mounted and axially displaceable on the trunnion through a given distance between a first axial position in which the bottom wall is spaced from a chamber end-wall, and a second axial position in which the bottom wall is in abutment with said end wall, and wherein a valve-equipped supply channel extending from a pressure medium source is connected to an opening in said end wall located opposite to the center hole in the bottom wall, for controlled delivery of pressure medium to the interior of the casing via the hole in the bottom wall thereof for transferring the casing from the first axial position to the second axial position while creating an over-pressure inside the casing.
In one preferred embodiment, a ring-shaped sealing device is disposed between said end wall and the bottom wall surface of the casing facing the end wall, wherein the sealing device forms a circular sealing line whose diameter is smaller than the diameter of that part of the trunnion surrounded by the casing.
Other advantageous embodiments will be apparent from the detailed description.
Because, in accordance with the invention, pressure fluid can be delivered to the interior of the casing surrounding the end of the trunnion, the casing will be pressed against the end wall primarily by the dynamic pressure from the fluid. In the case of a ring-shaped sealing device, the abutment pressure against the end wall will depend on how much smaller the diameter of the sealing line is than the diameter of the trunnion pressure surface. One beneficial circumstance is that the casing adapts its radial position through the position of the trunnion, and that the pressure of the casing against the end wall ceases when the supply of pressure medium is stopped, so that the casing can begin to rotate together with the trunnion, in the absence of friction losses between the casing and the end wall or trunnion.